Effect of Pressure on CO₂ Foam Displacements: A Micromodel Visualization Study

This paper presents the results of a visualization study on the effect of pressure on CO₂-foam displacements at pressures above and below the minimum miscibility pressure (MMP). CO₂-foam visualization experiments were performed by simultaneous injection of surfactant solution and CO₂ into a high-pressure, glass micromodel saturated with Maljamar crude oil. CO₂-foam was generated in situ during the simultaneous injection of surfactant solution and CO₂. Simultaneous injection of brine and CO₂ (WAG) displacements and pure CO₂ displacements were also conducted to provide a reference point for the CO₂-foam displacements. Results of these displacements were compared to examine the effects of pressure and injection mode on displacement performance and mechanisms. Two different kinds of micromodels were used in this study: modified-layer (MLAY) and modified-heterogeneous (MHET) micromodels. Pressures ranging from 775 to 1320 psia were investigated at a system temperature of 90°F. The surfactant solution used was 1% Alipal CD-128 in a 1 % NaCl brine.

Results show that sweep efficiencies were generally lower for pressures below the MMP. However, the effect of pressure on sweep efficiency for CO₂-foam displacements was less than for WAG displacements and much less than for pure CO₂ displacements. Sweep efficiencies for CO₂-foam displacements at pressures just below the MMP were as high as above the MMP. Due to the lower density of injected CO₂ near and below the MMP compared to that at higher pressures, the required mass of CO₂ was as little as one-third of that required at higher pressures for similar sweep efficiencies using CO₂-foam. These observations suggest the economical potential of operating CO₂-foam floods at pressures close to and just below the MMP.